The following code sample shows how to receive an IP multicast datagram.

#include <windows.h> 
#include <winsock2.h>

#define RECV_IP_ADDR      "234.5.6.7"     
#define DEST_PORT         4567            

int WINAPI WinMain (
              HINSTANCE hInstance,    // Handle to the current instance
              HINSTANCE hPrevInstance,// Handle to the previous instance
              LPTSTR lpCmdLine,       // Pointer to the command line
              int nCmdShow)           // Show state of the window.
{
  int index = 0,                      // Integer index
      iRecvLen;                       // Length of recv_sin
  char szMessageA[100];               // ASCII string 
  TCHAR szMessageW[100];              // Unicode string
  TCHAR szError[100];                 // Error message string

  SOCKET Sock = INVALID_SOCKET;       // Datagram window socket

  struct ip_mreq mreq;                // Used in adding or dropping 
                                      // multicasting addresses
  SOCKADDR_IN local_sin,              // Local socket's address
              recv_sin;               // Holds the source address on 
                                      // recvfrom function returns
  WSADATA WSAData;                    // Contains details of the 
                                      // Winsock implementation
    
  // Initialize Winsock. 
  if (WSAStartup (MAKEWORD(1,1), &WSAData) != 0) 
  {
    wsprintf (szError, TEXT("WSAStartup failed! Error: %d"), 
              WSAGetLastError ());
    MessageBox (NULL, szError, TEXT("Error"), MB_OK);
    return FALSE;
  }

  // Create a datagram socket, Sock.
  if ((Sock = socket (AF_INET, SOCK_DGRAM, 0)) == INVALID_SOCKET) 
  {
    wsprintf (szError, TEXT("Allocating socket failed! Error: %d"), 
              WSAGetLastError ());
    MessageBox (NULL, szError, TEXT("Error"), MB_OK);
    return FALSE;
  }

  // Fill out the local socket's address information.
  local_sin.sin_family = AF_INET;
  local_sin.sin_port = htons (DEST_PORT);  
  local_sin.sin_addr.s_addr = htonl (INADDR_ANY);

  // Associate the local address with Sock.
  if (bind (Sock, 
            (struct sockaddr FAR *) &local_sin, 
            sizeof (local_sin)) == SOCKET_ERROR) 
  {
    wsprintf (szError, TEXT("Binding socket failed! Error: %d"), 
              WSAGetLastError ());
    MessageBox (NULL, szError, TEXT("Error"), MB_OK);
    closesocket (Sock);
    return FALSE;
  }

  // Join the multicast group from which to receive datagrams.
  mreq.imr_multiaddr.s_addr = inet_addr (RECV_IP_ADDR);
  mreq.imr_interface.s_addr = INADDR_ANY;

  if (setsockopt (Sock, 
                  IPPROTO_IP, 
                  IP_ADD_MEMBERSHIP, 
                  (char FAR *)&mreq, 
                  sizeof (mreq)) == SOCKET_ERROR)
  {
    wsprintf (szError, TEXT("setsockopt failed! Error: %d"), 
              WSAGetLastError ());
    MessageBox (NULL, szError, TEXT("Error"), MB_OK);
    closesocket (Sock);
    return FALSE;
  }

  iRecvLen = sizeof (recv_sin);
  
  // Receive data from the multicasting group server.
  if (recvfrom (Sock, 
                szMessageA,
                100,         
                0,
                (struct sockaddr FAR *) &recv_sin,
                &iRecvLen) == SOCKET_ERROR)
  {
    wsprintf (szError, TEXT("recvfrom failed! Error: %d"), 
              WSAGetLastError ());
    MessageBox (NULL, szError, TEXT("Error"), MB_OK);
    closesocket (Sock);
    return FALSE;
  }
  else
  {
// Convert the ASCII string to a Unicode string
    szMessageA[99] = 0;
    MultiByteToWideChar(CP_ACP, 0, szMessageA, -1,
                                   szMessageW, sizeof(szMessageW)/sizeof(szMessageW[0]));
    MessageBox (NULL, szMessageW, TEXT("Info"), MB_OK);
  }

  // Disable receiving on Sock before closing it.
  shutdown (Sock, 0x00);

  // Close Sock.
  closesocket (Sock);

  WSACleanup ();

  return TRUE;
}

